Take diligent notes during the performance. Bring a pad and paper and an unobtrusive reading light, if needed. Write down any parts that were especially moving, thought-provoking, jarring or confusing. You may also want to keep a timeline of events to jog your memory.
Process your notes immediately. When the experience is fresh in your mind, go through the notes and expand any thoughts, clarify anything vague and write down any additional thoughts.
Build an outline. Organize your notes and important points into an outline. Dedicate one bullet point to each main idea and support it with two to three details.
Write a rough draft. Once you have an outline drawn out, begin writing your rough draft.
Give a brief description of the concert as a whole. This will serve as your introduction. Include any general impressions and bring up the points that you will delve into in greater detail. Mention whether you enjoyed the concert or not and give any relevant back story about the piece, the composer, the performers or the venue.
Discuss individual compositions. Note any musical themes, memorable features, the tone and style of the music and how (or if) the piece fit into an overall theme or program.
Evaluate the performance. Give your subjective opinion on which performers did particularly well or unsatisfactorily. Discuss dynamics, emotional projection and the performance of the conductor.
Write a conclusion. Sum up your findings and wrap up your report. Do not include any new information in the conclusion.